Micronesica 37(1):97-118, 2004 Green Turtles and Their Marine Habitats at Tinian and Aguijan, with Projections on Resident Turtle Demographics in the Southern Arc of the Commonwealth of the Northern Mariana Islands STEVEN P. KOLINSKI1,LARRY ITIBUS ILO2 AND JOHN M. MANGLONA2 1Department of Zoology, University of Hawaii 2538 The Mall, Edmondson 152, Honolulu, HI 96822 2Division of Fish and Wildlife, Department of Lands and Natural Resources, P.O. Box 10007, Lower Base Area, Saipan, MP 96950 Abstract—An estimated 351 individual green turtles were observed via 448 sightings in 27 surveys covering roughly 59% of the total shore and outer reef perimeter of Tinian, Commonwealth of the Northern Mariana Islands. Fourteen green turtles were observed during tow surveys cover- ing 95% of Aguijan’s shore and reef perimeter. No other sea turtle species were sighted. Juvenile turtles of various sizes dominated in all surveyed environments, and observations of turtles with estimated straight carapace lengths below 45 cm suggested recent and continuing recruitment at both Tinian and Aguijan. Twenty-four species of algae noted as green turtle forage in other regions of the world were identified at Tinian in this and previous surveys. Projections of turtle densities and abundances based on data from recent surveys of all southern-arc islands in the Commonwealth of the Northern Mariana Islands suggest the small uninhabited islands of Farallon de Medinilla and Aguijan sustain tens of turtles, while turtle numbers around the larger inhabited islands of Rota, Saipan and Tinian range from the very low to high hundreds. The Commonwealth portion of the southern arc is estimated to support between 1000 and 2000 main- ly immature resident green turtles. Turtle density and abundance appear highest at Tinian despite its smaller size relative to Saipan and its appar- ent paucity of potential seagrass forage. Introduction The distribution and status of sea turtles at many Pacific island localities has yet to be determined, but is of concern as continuing human expansion and coastal development throughout the region has great potential to negatively impact local sea turtle populations through increased harvests, incidental catch, and the degra- dation of nesting and critical nearshore and pelagic habitats (see Lutcavage et al. 1997, Bjorndal 1997, NMFS & USFWS 1998a-d). The lack of specific island and archipelago information hinders efforts to understand not only local, but also the 98 Micronesica 37(1), 2004 large-scale regional dynamics of turtle populations, and reduces the ability to effectively plan development and human activities to minimize impacts and to manage sustainable utilization of turtles as a resource. Recent efforts to investigate and document sea turtle activities within the United States Commonwealth of the Northern Mariana Islands (CNMI) (McCoy 1997, Pultz et al. 1999, Kolinski et al. 1999, 2001, Dollar & Stefasson 2000, Ilo & Manglona 2001, 2002) provide preliminary baseline information that is rele- vant to estimating and understanding turtle demographics within the region. Such information is likely to be key to decision makers and managers as island populations and tourist development expands, and local public and political interest continues in gaining legal exemption under the U.S. Endangered Species Act to allow for traditional harvests of sea turtles (see McCoy 1997). Although far from complete, evidence to date suggests very limited turtle nesting on CNMI shores (Pritchard 1977, 1982, Johannes 1986, Wiles et al. 1989, 1990, D. Grout unpubl., McCoy 1997, Pultz et al. 1999, Kolinski et al. 2001). Anecdotal reports of turtles in nearshore marine habitats exist (Pritchard 1977, 1982, Johannes 1986, Wiles et al. 1989, 1990, McCoy 1997), however focused assess- ments of the resident sea turtle population(s) are only recently being made (Pultz et al. 1999, Kolinski et al. 1999, 2001, Dollar & Stefasson 2000, Ilo & Manglona 2001, 2002). The present study expands the focus on assessing the status of resident green turtles (Chelonia mydas) and their habitats within CNMI waters. Data specific to the islands of Tinian and Aguijan are presented, and a summary of available infor- mation from known surveys within the region was used to construct a projection of present day turtle demographics for islands within the southern arc of the archipelago. Study Area The Mariana Archipelago has 15 islands and various submerged banks and is oriented south to north from 13º to 20º5'N and 144.5º to 146ºE (Figure 1). The Philippine Sea borders the western shores and the Pacific Ocean the east. Islands of the Marianas are located on two separate arcs. An inner, northern arc supports the volcanically active or recently active islands of Farallon de Pajaros (Uracas), Maug, Asuncion, Agrihan, Pagan, Alamagan, Guguan, Sarigan and Anatahan (the Northern Marianas). Islands on the frontal, southern arc are capped or surround- ed by limestone terraces and include Farallon de Medinilla, Saipan, Tinian, Aguijan, Rota and Guam (the Southern Marianas; Wells & Jenkins 1988, Birkeland 1997). The archipelago is divided politically between the United States territory of Guam and the United States Commonwealth of the Northern Mariana Islands. Approximately 69,000 humans inhabited the CNMI as of 2000, with more than 99.99% found in the Southern Marianas, the vast majority living on Saipan (Evans et al. 2002). Kolinski et al.: Green turtles at Tinian 99 TINIAN ISLAND Tinian (15º00'N, 145º38'E) is a raised, relatively flat 102 km2 limestone island reaching heights of 170 m (Figure 1; Eldredge 1983, Wells & Jenkins 1988). It is located 5 km southwest of Saipan and 9 km northeast of Aguijan. The majority of shoreline consists of low to high limestone cliffs with sea-level caverns, cuts, notches and/or slumped boulders, commonly bordered by intertidal benches (Doan et al. 1960, Eldredge & Randall 1980, Eldredge 1983). Thirteen beach areas have been defined (Pultz et al. 1999), 10 at west coast locations and 3 (one distinct and Figure 1: The Mariana Archipelago and Tinian and Aguijan Islands, CNMI 100 Micronesica 37(1), 2004 two discontinuous beach complexes) along the east coast. Beach deposits consist mainly of medium to coarse grain calcareous sands, gravel and rubble interspersed amongst exposed limestone rock (Doan et al. 1960). All beaches reportedly sup- port turtle nesting activities (Wiles et al. 1989, Pultz et al. 1999). The north, east, and south coasts of Tinian have very limited fringing or apron reef development that is most conspicuous at Unai Dangkolo (Doan et al. 1960, Eldredge & Randall 1980, Maragos 1985). Submarine topography is main- ly characterized by limestone pavement with interspersed coral colonies and occa- sional zones of submerged boulders. Coral reef development is more prevalent at various west coast locations, with fringing coral reef habitats present inside Lamanibot and Peipeinigul Bays. A patch and small barrier reef system (altered as a breakwater for the harbor) is located within the Tinian Harbor area (Doan et al. 1960, Eldredge & Randall 1980, Eldredge 1983, Maragos 1985). Human development is concentrated mainly along the west coast at San Jose. The 2000 census identified 3540 residents (Evans et al. 2002). Tourism and agriculture are the main industries. AGUIJAN ISLAND Aguijan (14º51'N, 145º34'E) is a raised, steeply cliffed, 168 m high, nearly flat-topped limestone plateau approximately 7.2 km2 located 9 km southwest of Tinian (Figure 1; Eldredge 1983, Wells & Jenkins 1988). Its nearest neighbor to the southwest is Naftan Rock (1 km) and beyond that, 90 km to the south, is Rota. There are no sand beaches and bench development is limited (Eldredge & Randall 1980). Benthic topography is characterized by steeply sloping limestone pave- ment with scattered boulders and limited coral development around most of the island (Tsuda et al. 1971, Eldredge & Randall 1980, Maragos 1985, Randall 1985, Wells & Jenkins 1988). Along the east coast an 18 m deep pavement platform extends roughly 300 m offshore and contains scattered corals, grooves and sand deposits. Along the west coast a topographically diverse platform connects Aguijan to Naftan Rock. This platform is characterized by limestone rock, live coral and sand, and reaches depths of from nine to 18 m. There is no permanent human settlement on the island, however the nearshore waters are regularly fished by CNMI residents. Methods SEA TURTLE ASSESSMENTS The nearshore marine habitats of Tinian were surveyed from 12–21 March 2001 by investigators of the University of Hawaii, the Saipan Division of Fish and Wildlife and the Tinian Department of Land and Natural Resources. Aguijan was surveyed on 20 March 2001. Various members of the community, including Department of Land and Natural Resources representatives, local fishermen and other local observers, provided information regarding notable sea turtle habitats believed worthy of investigation (which included most of the nearshore marine Kolinski et al.: Green turtles at Tinian 101 habitat surrounding Tinian Island). Methods for turtle and marine habitat assess- ments were similar to those described in Kolinski et al. (2001). Approximately 59% of Tinian’s, and 95% of Aguijan’s outer reef and shoreline perimeter were examined using one or more of the following three methods: 1. Tow surveys were conducted along portions of the north, south and the major- ity of the west coast of Tinian and all of Aguijan, as permitted by ocean con- ditions. Two boats (a 9.4 m Fountain and a 4.5 m Whaler) were used to sur- vey parts of the north and west regions of Tinian. Two to three people using snorkeling gear were surface towed, with the Whaler surveying shallower environments maintaining distances from shore approximately 20 to 100 m, and the Fountain surveying deeper environments 30 to 50 m offshore of the Whaler. Surveyed depths ranged from two to roughly 70 m (estimated limit of visibility). When a turtle was sighted the boat was stopped and the species, size, sex (when discernable), activity, time, depth and habitat characteristics were relayed to a recorder on the boat.